1. Field of the Invention
The present invention relates generally to the testing of electronic components such as chips for functional reliability prior to assembly by procedures such as burn-in procedures and, in particular, to liquid compositions which are used as a thermal interface between a heatsink and a chip during the test procedure which compositions enhance the thermal conductivity between the heat sink and the chip, are easily removed from the heat sink and the chip after the test procedure without any deleterious residue and which allow the use of high temperatures for extended periods during the test procedure without any significant degradation of the composition and to a method for using the compositions in electronic component test procedures.
2. Background of the Invention
Due to the complexity of high performance MCMs fabrication processes using high power devices, it is important to pretest the device performance for functional reliability prior to assembly in order to eliminate/minimize module rework and reduce overall product production cost. Test and Burn-in processes for high density device chips require extended Test/Burn-in duration at relatively high temperature to evaluate and assure long term reliability of device performance.
One of the commonly used methods for Known Good Die testing (KGD) for solder ball flip-chip interconnection is based on temporary chip attachment (TCA) which involves placing a heatsink on the back of the device mounted on the temporary chip carrier with a cooling medium or thermal interface material interposed between the heatsink and the silicon chip for effective heat dissipation during extended Burn-in.
U.S. Pat. Nos. 5,918,665 and 6,577,146, which are incorporated herein by reference, disclose burn-in procedures for testing integrated circuit chip packages. As discussed in the patents, the thermal resistance between the chip surface and the heat sink surface is important to the reliability of the test procedure and it is typical to use a liquid film between the chip surface and the heat sink surface to improve the thermal conductivity between the mating faces. It is very important that the liquid film have a high conductivity and a high stability over an extended period of time at elevated temperatures, that the liquid not degrade significantly during the test procedure and that the liquid be easily removed from both surfaces after the test procedure is completed.
Several options for the cooling media are available with a range of properties to meet the functional requirement during test and burn-in which include low thermal resistance organic liquids, reworkable solid thermal interface, PCM, thermal grease, thermally conducive pads and tapes, etc. An important consideration for the thermal interface in the TCA method for KGD is its cleanability from the die backing and the heatsink after test and burn-in so as to obtain residue-free surfaces with no impact on the reuse performance of the heatsink and the follow-on device chip assembly process. To minimize manufacturing cost and down time, it is also required that heatsink cleaning frequency is kept to a minimum by continuing its use for multiple manufacturing Test/Burn-in cycles which of course would depend on the chemical stability of the thermal interface material.
With high circuit density devices requiring extended duration conditions, Liquid Thermal Interface (LTI) is preferred for maintaining close interfacial contact between the device chip and the heatsink which is critical for efficient heat dissipation from the device to the heatsink during Test/Burn-in program. For effective heat transfer from the device chip to the heatsink using LTI material, it is important that it has good wetting with the contacting surfaces so that a void-free interface is obtained. In the case of conductive filler carrying thermal interface materials such as thermal paste or greases, contact at the interface is subject to filler size in addition to the likelihood of particulates causing voids at the interface resulting in increase of interfacial resistance during Test/Burn-in process.
Among the various options for the Liquid Thermal Interface materials, Polyalphaolefin Oils (PAOs), are preferred thermal interface materials between electronic components such as high density Si device chips and high thermal conductivity metal heat sinks because of their highly desirable properties of high thermal conductivity (about 0.18 W/mK), lower thermal resistance than common liquids, availability in different viscosity grades with high purity, high thermal stability, low volatility, no significant health and safety or toxicity issues, and no environmental emission concerns. Other materials which are similar to PAOs may be used in the invention but PAOs are preferred because of their demonstrated effectiveness and the following description will be directed to these materials for convenience.
A commercial PAO composition uses PAO 100 which is formulated with Irganox 1010 antioxidant [tetrakis-(methylene-3,5 di-tert-butyl-4-hydroxy-hydrocinnamate)methane] as LTI for chips requiring relatively short duration Test/Burn-in, typically 24 hrs. at 120° C. and 140° C. However, for devices requiring high temperature extended duration conditions prior to assembly to meet the functional reliability requirement for present semiconductor product programs, the commercial fluid was found to be unacceptable for 144 hrs and longer at 140° C. This formulation is subject to thermo-oxidative chemical changes during extended exposure to air environment at these temperatures resulting in formation of reactive by-products including radicals, peroxides and hydroperoxides, which in turn polymerize and form non-removable deposits depleting the antioxidant and degrading the performance of the interface liquid. Apart from the problem of inadequate thermal cooling performance, the interfacial polymerization reaction products cause sticking of the heatsink with the chip, making it difficult to effectively clean the chip back side and the heatsink to obtain residue-free surfaces for follow-on processing.